Various thermal treatment systems have been and continue to be used to separate volatile from non-volatile substances. For example, thermal desorption units are commonly used to remove substances such as mercury and volatile organics from soil and other solids. These prior art systems use heated rotating or non-rotating vessels operated under slight vacuums of less than 1 inch of mercury in the rotating systems to high vacuums of over 28 inches high in the non-rotating systems. The slight vacuum employed in rotating systems is to prevent leakage of environmentally-regulated substances out of the retort and off-gas treatment system, while the high vacuum in non-rotating systems serves to shorten process times. Although the technology is well known, there are several drawbacks and limitations which are overcome with the current invention.
First, in the prior art rotating systems, the off-gas treatment equipment, which is required to remove contaminated particulates and regulated chemicals prior to discharge of the treated gases to the atmosphere, is very large and expensive compared to the system's processing rate. Due to ever more stringent air emission regulations and the need to protect human health and the environment, these off-gas treatment systems will continue to become more sophisticated and costly. One of the primary reasons that the off-gas processing systems associated with prior art thermal units are so large and expensive is because of the high volume of carry out of contaminated particulates and combustion, sweep and/or leakage gases exhausted from the retort during operation. To reduce the size and complexity of the off-gas treatment systems, indirectly fired retort vessels are used.
Heat is usually applied to the outside of the retort or applied with resistance heaters inside of the retort. These systems reduce the amount of particulates and eliminate the combustion gases exiting the retort. The prior art systems, however, do not eliminate the carry out of particulates from the retort and they still require a relatively large amount of sweep gas to move the vaporizing chemicals out of the retort. Therefore, even though an improvement, prior art indirectly fired retorts still require relatively large and expensive off-gas treatment systems.
In the present invention, however, the combination of a substantial vacuum ranging from 1 to 29+inches of mercury and an internal filter virtually eliminates carry out of particulates and minimizes the need for sweep gases to move the vaporized substances out of the retort. The substantial vacuum of the present invention provides at least the following two benefits. One; it minimizes the suspension of fine particulates within the retort thereby minimizing the dust loading to the internal filter and two; it establishes a pressure gradient to assist in the diffusion of gases out of the retort. The amount of particulates and sweep gases exhausted from the retort is thereby reduced to the lowest practical level achievable. The dramatic reduction in off-gas treatment equipment size, complexity and cost is a significant improvement over prior art technology.
Additionally, there are many cases in which one or more of the components of the matrix and/or the substances to be separated are thermally sensitive. That is, one or more of the substances break down to unwanted substances and/or the structure of one or more matrix components are altered in a way that adversely affects subsequent treatment or reuse. Prior art systems employing heat and vacuum can be used for these situations. The use of vacuum lowers the boiling point of substances and, depending upon the substances involved, may allow the separation of chemicals at below critical temperatures. However, prior art systems that use above ambient temperature and substantial vacuum do not rotate and are therefore small batch systems requiring long process times. Prior art systems do not use rotation in combination with high temperature and substantial vacuum because of difficulties with sealing a hot rotating vessel under those conditions. Inadequate seals allow uncontrolled amounts of air to flow into the retort resulting in carry out of particulates and adding to the off-gas treatment requirements. To overcome the low processing rates of the prior art systems, the present invention employs a heated rotating vessel operating under a significant vacuum.
Rotation increases the processing rate by improving heat transfer to the process material, minimizing the path length the volatilized substances have to travel to exit the retort and minimizing the interference caused by collisions between the volatilized substances and particulates prior to exiting the retort. The present invention overcomes the problem of sealing a heated rotating vessel under substantial vacuum through the use of a uniquely designed rotating sleeve and externally mounted seal configuration that allows the seals to be easily cooled and maintained below the maximum operating temperature of the seal.
Another problem associated with the prior art systems is that the common materials available for construction of the retort limit the maximum operating temperature of an indirectly fired unit. Therefore, substances with boiling points above these temperature limits can not be rapidly volatilized within these units but must be processed using expensive internally heated refractory lined units. In the present invention, however, the use of a high vacuum within a rotating vessel lowers the boiling point of substances up to several hundred degrees Fahrenheit. This improvement allows the present invention to volatilize materials with high boiling points at a lower cost than has been possible using the prior art.
Finally, cleanup levels at contaminated sites are often dictated by the best available demonstrated technology. Often, if acceptable cleanup levels can not be achieved from the standpoint of the protection of human health and the environment with on-site treatment, contaminated material must be excavated and hauled to a site to be treated and buried. The reason other technologies do not achieve an acceptably low residual level of contamination in the processed material is often because contaminants from deep within the material particulates is not removed. Over time, "clean" sites can be found to be contaminated because chemicals within processed material have migrated to the surface of the material particulates and are once again detected in analytical tests. The use of a high vacuum in the present invention provides the benefits of accelerating the overall separation process and maximizing the diffusion of volatile compounds from within the solid to the surface where they are rapidly vaporized and exhausted from the retort. The diffusion is maximized by the increased pressure gradient between the center of a particle and the surface of the particle. The contaminant within the particle vaporizes producing a high relative pressure inside the particle compared to the very low pressure at the surface of the particle. The present invention therefore accomplishes the goal of producing the lowest residual contamination levels achievable by efficiently removing contaminants from deep within the solid matrix.
Prior methods and apparatus for processing materials are disclosed in U.S. Letters Pat. Nos. 4,268,306; 5,183499; 5,244,492; and 5,300,137.
The present invention provides many advantages and eliminates several deficiencies found in the prior art thermal processing systems. As such, the invention meets a long-standing need to provide a novel thermal processing apparatus that 1) promotes the efficient separation of volatile and non-volatile substances, 2) promotes the efficient separation of volatile substances one from another and 3) promotes various chemical reactions through the use of a high throughput unit capable of mobile operation that a) reduces off-gas treatment requirements to the lowest practical levels achievable, b) produces the lowest practical residual contamination levels within the processed material, c) separates thermally sensitive substances and d) lowers the boiling point of substances to overcome temperature limitations imposed by the retort materials of construction. These improvements over prior art are accomplished by the combination of an externally or internally heated rotating vessel with or without an internal filter and blow back system operating under a vacuum of greater than one inch of mercury.